Noise reduction in a travelling wave tube employing a helix input coupler



Sept. 10, 1968 c 31 3,491,29

NOISE REDUCTION IN A TRAVELLING WAVE TUBE EMPLOYING A HELIX INPUT COUPLER Filed July 28, 1965 SEE 35 INVENTOR (YR/1. HENRY D/x ATTORN 5 United States Patent-O I ABSTRACT OF THE DISCLOSURE A helix type travelling wave tube wherein an input signal is launched onto a slow wave structure helix by means of an input coupler comprising a wire helix coaxially surrounding a further wire helix connected in series with the slow wave structure helix between an electron gun and the nearer end of the slow wave structure helix, the dimensions of the further helix differing from those of the slow wave structure helix so that noise on the electron beam at frequencies within the range of frequencies over which the tube is required to operate is not amplified over the length of the further helix.

This invention relates to travelling wave tubes.

The invention relates particularly to travelling wave tubes of the kind comprising an evacuated envelope in which is housed an elongated slow wave structure comprising a wire helix along which an electromagnetic wave may be propagated in a forward wave mode, and an electron gun for projecting an electron beam along the length of said structure so as to interact with a wave propagated along said structure.

It has been proposed to apply an input signal to such a tube by means of an input coupler comprising a wire helix coaxially surrounding a portion of the slow wave structure adjacent the electron gun. With regard to-noise performance, however, such an arrangement has the disadvantage that while the input signal is not amplified until the end of the input coupler remote from the electron gun, noise signals are amplified from the end of the slow wave structure nearer the electron gun.

It is an object of the present invention to provide a travelling wave tube of the kind specified wherein this disadvantage is overcome.

According to the invention, in a travelling wave tube of the kind specified there is provided, between the electron gun and the end of the slow wave structure nearer the electron gun, a further wire helix disposed coaxial with and connected in series with the helix of the slow wave structure, and an input coupler comprising a wire helix disposed coaxially surrounding said further wire helix, the arrangement being such that over a band of frequencies an input signal may be launched from the input coupler onto the slow wave structure via said further helix, and such that at any frequency in said band the respective phase velocities of propagation of electromagnetic waves along said further helix and the slow wave structure differ sufiiciently to ensure that when the electron beam has a velocity such that it can interact with an electromagnetic wave propagated along the slow wave structure, there can be no substantial interaction between the electron beam and an electromagnetic wave of the same frequency propagated along said further helix.

Preferably, said further wire helix has the same diameter as, but a different pitch from, the helix of the slow wave structure so that the two helices effectively form sections of a single helical structure of uniform diameter. The helical structure preferably also includes a transition 3,401,298 Patented Sept. 10, 1968 section in which the pitch varies monotonically with length between values corresponding to the pitches of said two helices; in order to obtain a good match over abroad band of frequencies the variation of the pitch in the transition section is preferably non-linear with the highest rate of variation at the end of the transition section adjacent the helix of the slow wave-structure. With such an arrangement the helical structure may be supported directly within a tubular portion of the envelope of electrically insulating material, the helix of the input coupler closely surrounding said tubular portion.

One travelling wave tube in accordance with the in vention and suitable for operation over the frequency band 200 mc./s. to 1,000 mc./s. whilst exhibiting good noise and limiting performance over this frequency range will now be described by way of example with reference to the accompanying drawings in which:

FIGURE 1 is a part sectional view of the travellin wave tube; and

FIGURE 2 is an enlarged diagrammatic view of part of the tube; and

FIGURE 3 is an end view of FIGURE 2.

. Referring now to FIGURE 1, the tube includes an evacuated tubular glass envelope 1 comprising two communicating coaxial sections 2 and 3 of lengths 2.5 inches and 11.5 inches respectively. The section 2 houses an electron gun 4 arranged to project an annular electron beam coaxially through a slow wave structure, comprising part of a helical wire structure 5 housed in the section 3, towards a collector electrode 6 sealed into the end of the section 3 remote from the section 2.

The electron gun 4 is of conventional form and includes an indirectly heated ring-shaped cathode 7 provided with an active coating by a suitable coating technique, a frusto-conic'al beam forming electrode 8 and four coaxially aligned spaced apart apertured disc-shaped accelerating electrodes 9, 10, 11 and 12. A thin-walled tubular metal member 13 of length 0.220 inch 'and internal diameter 0.203 inch extends coaxially from the side of the final accelerating electrode 12 remote from the cathode 7. Leads (not visible) from the electron gun 4 are sealed through a plug of insulating material (not visible) sealed into the free end of the section .2. The electron beam produced by the gun has a mean diameter of 0.110 inch and a thickness of .027 inch, and is focussed by means of an electromagnetic 'focussing coil,

(not shown) disposed around the outside of the tube in operation, which produces a uniform magnetic field of 450 gauss directed parallel to the axis of the envelope 1.

Referring now also to FIGURES 2 and 3, the section 3 has a wall thickness of 0.025 inch and a nominal internal diameter of 0.230 inch, and at equally spaced positions around its internal surface there are formed three axially extending ribs 14, the inner surfaces of the ribs 14 lying on a circumference of diameter 0.208 inch, and each rib 14 having a width of 0.06 inch.

The helical structure 5 is formed from 0.005 inch diameter molybdenum wire and has 'an internal diameter of 0.208 inch, each turn of the structure 5 being partially embedded in the ribs 14 so that the structure 5 is rigidly supported within the section 3, the structure 5 being otherwise spaced from the internal surface of the section 3. t

The helical structure 5 has an overall length of 11 inches and comprises in series, starting from the end nearest the electron gun 4; an input coupling section 15 of length 2.813 inches and having a pitch of 33 turns per inch; a first transition section 16 of length 0.188 inch in which the pitch changes exponentially from 33 to turns per inch with the highest rate of change remote from the input coupling section 15; an interaction section 17 of length 6.625 inches and having 'a pitch of 100 turns per inch; a second transition section 18 similar to the transition section 16, but in which the pitch decreases instead of increases; and an output coupling section 19 similar to the input coupling section 15, but having a length of 1.188 inches.

The end of the helical structure nearest the electron gun 4 is electrically connected to the free. end of the metal tube 13 the end of the interaction section 17 being at a distance of 3 inches from the accelerating electrode 12.

The tube further includes two tubular-shaped attenua tors 20 and 21 (shown in outline only) which fit tightly around the outside of the part of the section 3 in which the interaction section 17 is housed, each attenuator 20 or 21 comprising a wire helix formed of 0.004 inch diameter tinnedcopper wire wound in the opposite sense to the helical structure 5, a layer of carbon filled paper wrapped tightly around the outside of the wire helix, and a length of thermosetting polyester adhesive tape wound tightly around the inside of the wire helix and the outside of the carbon-filled paper layer. In the attenuator 20 the helix has a length of 0.75 inch and a pitch of 50 turns per inch, this attenuator 20 being disposed with its end nearer the electron gun 4 at a distance of 2 inches from the corresponding end of the interaction section 17; in the other attenuator 21 the helix has a length of 0.5 inch and a pitch of 50 turns per inch, this attenuator 21 being disposed with its end nearer the electron gun 4 at a distance of 5 inches from the corresponding end of the interaction section 17.

This section 3 also carries three closely fitting tubular copper sleeves 22 having a wall thickness of 0.004 inch, one sleeve 22 extending from the end of the attenuator 20' nearer the electron gun 4 for a distance of 2 inches towards the electron gun 4, a second sleeve 22 extending between the two attenuators 20 and 21 and the third sleeve 22 extending from the end of the attenuator 21 nearer the collector electrode 6 for a distance of 0.875 inch towards the collector electrode 6.

The tube also includes a third attenuator 23 of similar construction to those described above, the attenuator 23 being disposed around the end of the helical structure 5 adjacent the collector electrode 6, and incorporating a double-wound helix (not visible) of length of 0.3 inch and pitch 16 turns per inch formed from 0.004 inch diameter tinned copper wire.

An input signal is supplied to the tube via a coupler 24 comprising a wire helix (not visible) of'length 0.625 inch and pitch 18 turns per inch formed from 0.036 inch diameter copper wire and wound in the opposite sense to the helical structure 5. The coupling helix is housed within and spaced from a short metal tube 25'and is disposed around the part of the envelope 1 housing the input coupling section 15 of the helical structure 5. An output signal is derived from the tube via a similarly constructed output coupler 26 disposed around the part of the envelope 1 in which the output coupling section 19 of the helical structure 5 is housed.

The section 3 together with the associated attenuators 20, 21 and 23, copper sleeves 22 and input and output couplers 24 and 26 is housed coaxially within a tubular metal enclosure 27 one end of which abuts against the shoulder between the sections 2 and 3 and the'other end of which lies about 2 inches beyond the collector electrode 6. At a position adjacent the collector electrode 6 the metal enclosure 27 carries a bush of magnetic material 28 which serves as a pole piece for the magnetic field required to focus the electron beam. A second annular pole piece 29 for this field is coaxially disposed around the envelope 1 adjacent the electron gun 4.

The metal enclosure 27 and the section 2 are coaxially surrounded by a tubular metal casing 30 which extends between a metal boss 31 at the end of the metal enclosure 27 nearest the collector electrode 6, to which boss 31 a lead 32 from the collector electrode 6 is electrically connected, and a conventional valve base 33 disposed adjacent the electron gun 4 and to the pins 34 of which the leads (not visible) from the electron gun 4 are connected.

Coaxial leads 35 from the input and output couplers 24 and 26 extend along the tube via the space between the metal enclosure 27 and the metal casing 30, through apertures (not visible) formed in the pole piece 28 and through further apertures (not visible) in the metal boss 31. The inner of each coaxial lead 35 is connected at the relevant end to that end of the appropriate coupling helix 24 or 26 remote from the interaction section 17 and the corresponding end of the outer is connected to the metal tubular member 25, in which that coupling helix is housed. At their other ends the coaxial leads 35 are provided with conventional coaxial sockets 36.

In operation of the tube, with the electron beam having a meanyelocity corresponding to a voltage of 35 volts and a current density of 5 milliamps per square centimetre, a gain of between 20 dbs. and dbs. is obtained over a bandwidth of from 250 megacycles per second to 1,000 megacycles per second. A noise figure of less than 10 dbs. is obtained over this frequency range with the beam voltage adjusted for minimum noise, and the saturated power output of the tube is in the range l3 dbm to --3 dbm, this output being maintained over a 40 dbs. range of input power variation.

It will be understood that the low noise figure is in part attributable to the fact that, due to the difference in the phase velocity of propagation of electromagnetic waves between the interaction and input coupling sections 17 and 15, noise is not amplified until the beginning of the interaction section 17, that is, the same point at which amplification of the input signal begins. The length of the input coupling section 15 thus effectively constitutes part of a field free drift length, which is also partly constituted by the length of the metal tube 13, this drift length being chosen, in accordance with known considerations, to improve the performance of the tube in respect of noise.

The transition sections 16 and 18 are included to give a good match between the coupling and interaction sections 15, 19 and 17, an exponential change in pitch having been found most suitable for this purpose.

The two attenuators 20 and 21 surrounding the helical structure 5 serve to prevent oscillation in known manner, two attenuators rather than one being used to assist in reducing reflections which tend to occur at the output end of the attenuator and at the ouput coupler if only one attenuator is used. I

The attenuator 23 adjacent the collector electrode serves to absorb any electromagnetic wave energy not extracted via the output coupler 26; this is found necessary since the output coupler 26, being open circuit at its end nearer the collector electrode 6, tends to give rise to reflections. The input coupler 24 is not found to suffer from this difiiculty because it is terminated by the tube 13.

The copper sleeving 22 surrounding the major portion of the interaction section 17 serves to decrease the dispersive properties of the interaction section 17 at the low frequency end of the operating band of the tube, thus improving the flatness of the frequency/gain characteristic of the 'tube. The presence of the sleeving 22 is also found to increase the gain of the tube at the high frequency end of the operating band when the tube is operated so as to obtain a minimum noise figure. This is thought to be due to the fact that under least noise conditions the beam velocity is nominally slightly less than the phase velocity of the wave propagated along the interaction section 17 and the effect of the sleeving 22 is to lower the latter velocity at higher frequencies. In addi tion the sleeving 22 is found to improve the limiting performance of the tube.

I claim:

1. A travelling wave tube comprising: an evacuated envelope in which is housed an elongated slow wave structure including a wire helix along which an electromagnetic wave may be propagated in a forward wave mode; an electron gun for projecting an electron beam along the length of said structure so as to interact with a wave propagated along said structure; a further wire helix disposed coaxial with and connected in series with the helix of the slow wave structure between the electron gun and the end of the slow wave structure nearer the electron gun; and an input coupler comprising a wire helix disposed coaxially surrounding said further wire helix so that over a band of frequencies an input signal may be launched from the input coupler onto the slow wave structure via said further helix; the dimension of the further helix and the helix comprising the slow wave structure being different so that at any frequency within said band the phase velocities of propagation along the further helix and the slow wave structure differ sufliciently to ensure that when the electron beam has a velocity such that it can interact with an electromagnetic wave propagated along the slow wave structure, there can be no substantial interaction between the electron beam and an electromagnetic wave of the same frequency propagated along said further helix.

2. A travelling wave tube according to claim 1 in which there is provided beyond the end of the slow wave structure remote from the electron gun an additional wire helix disposed coaxial with and connected in series with the helix of the slow wave structure, and an output coupler comprising a wire helix disposed coaxially surrounding said additional wire helix.

3. A travelling wave tube according to claim in which said further wire helix has the same diameter as but a different pitch from the slow wave structure so that the slow wave structure and said further wire helix effectively constitute different sections of a single helical wire structure of uniform diameter.

4. A travelling wave tube according to claim 3 in which said helical wire structure also includes a transition section between the slow wave structure and said further wire helix, the transition section having a pitch which varies monotonically with length from a value at the end adjacent the slow wave structure substantially equal to the pitch of the slow wave structure to a value at the end adjacent said further helix substantially equal to the pitch of said further wire helix.

5. A travelling wave tube according to claim 4 in which the pitch of the transition section varies nonlinearly with the highest rate of variation at the end of the transition section adjacent the slow wave structure.

6. A travelling wave tube according to claim 5 wherein the pitch of the transition section varies exponentially.

7. A travelling wave tube according to claim 3 in which said helical wire structure is supported directly within a tubular portion of the envelope which consists of an electrically insulating material, the input coupler surrounds the part of said tubular portion which houses said further wire helix, and there are provided at least two attenuators fitting around different sections of the part of said tubular portion which houses the slow wave structure.

8. A travelling wave tube according to claim 4 wherein there is provided beyond the end of the slow wave structure remote from the electron gun an additional wire helix disposed coaxial with and connected in series with the helix of the slow wave structure, and an output coupler comprising a wire helix disposed coaxially surrounding said additional wire helix being of substantially the same dimensions as said further wire helix and being connected to the slow wave structure via a transition section of substantially the same dimensions as the transition section between the slow wave structure and said further wire helix.

9. A travelling wave tube according to claim 8 wherein the slow wave structure, the further and additional wire helices and the transition sections form sections of a single helical wire structure of uniform diameter which is supported directly within a tubular portion of the envelope of electrically insulating material with the input and output couplers surrounding the parts of said tubular portion housing said further and additional wire helices respectively, and there is provided an attenuator fitting around a part of said tubular portion housing a part of the helical structure lying on the side of the output coupler remote from the electron gun.

References Cited UNITED STATES PATENTS 2,672,571 3/1954 Harman 3153.6 X 2,767,259 10/1956 Peter 315-36 X 2,908,844 10/1959 Quate 315-3.6 3,092,750 6/1963 Haus et al. 3153.6 3,258,706 6/1966 Sturrock 3153.6 X 3,293,482 12/1966 Wolkstein 315-3.6 X

FOREIGN PATENTS 696,058 8/1953 Great Britain.

ELI LIEBERMAN, Primary Examiner.

S. CHATMON, JR., Assistant Examiner. 

